The present invention relates to a transmission system comprising a transmitter coupled via a transmission channel to a receiver, said transmitter being arranged for transmitting a main signal and an auxiliary signal to a receiver, which is arranged for receiving the main signal and the auxiliary signal.
The present invention also relates to a transmitter and a receiver for use in such a transmission system. The invention further relates to a transmission method, and a signal.
In transmission systems it is often desired to transmit in addition to a main signal an auxiliary signal. The main signal can e.g. be a speech signal to be transmitted via a radio link of a mobile telephone system. The auxiliary signal can be e.g. a control signal for requesting a reconfiguration of a the receiver to adapt it to a change of the signal it should receive.
It is possible to include a special field in the transmission frame for such auxiliary signal, but this inclusion of a special field in the transmission frame is very inefficient if the auxiliary signal is rarely used. It is also possible to use a frame structure which is changed when the auxiliary signal has to be transmitted. The use a variable frame structure results in a substantial increase of the complexity of the transmission systems.
An object of the present invention is to provide a transmission system according to the preamble in which the above mentioned inefficiency and added complexity are avoided.
To achieve said objective, the transmission system according to the invention is characterized in that the transmitter comprises an encoder for encoding the main signal in a way described by a coding property, in that the receiver comprises a decoder for decoding the main signal in a way described by the coding property, in that the transmitter comprises coding property sequencing means for changing the coding property according to a predetermined sequence dependent on the auxiliary signal, and in that the receiver comprises a sequence detector for detecting said predetermined sequence in the coding property.
In the transmission system according to the invention use is made of the fact that the main signal is often encoded. In general the main signal will be encoded according to a source coding scheme in order to reduce its bitrate, and/or to a channel encoding scheme to enable reliable transmission of the main signal over the transmission channel. The source coding scheme can involve the use of CELP coding for a speech signal, and the source coding can involve the use of error correcting codes such as a convolutional coder or a Reed-Solomon block code. The coding property can be the output bitrate of a speech encoder, or the rate of a convolutional encoder. In some transmission systems, the coding property can be changed on the fly to respond to a change of the capacity of the transmission channel. This capacity of the transmission channel can change due to a increased or decreased signal strength and/or interference received from a radio link, or due to more or less congestion of a transmission network such as the Internet.
By changing the coding property by purpose according to a predetermined sequence, it is possible to transmit the auxiliary signal to the receiver without reserving extra space in the frame structure or having to use a varying frame structure.
An embodiment of the present invention is characterized in that the transmission system comprises transmission quality determining means for determining a transmission quality of the transmission channel, and adaptation means for adapting the coding property in dependence on the transmission quality, and in that the coding property sequencing means are arranged for changing the coding property only to values corresponding to a lower transmission quality than the transmission quality determined by the transmission quality determining means.
According to this measure, it is prevented that due to the actions of the coding property sequencing means the coding property is chosen in such a way that reliable transmission is not possible anymore. This could happen when the coding property sequencing means switch to a coding property suitable only for a transmission quality better than the present transmission quality.